The present invention relates to an electro-optical distance meter, which projects a modulated measuring light to an object to be measured, receives a reflected measuring light from the object to be measured and measures a distance to the object to be measured by a phase difference between the measuring light and the reflected measuring light.
In the electro-optical distance meter which perform a distance measurement by detecting the phase difference between the measuring light and the reflected measuring light, a distance as measurable and a measurement accuracy are determined by a modulated frequency. For this reason, in order to make measuring from a near distance to a long distance possible, it is necessary to project a distance measuring light with a plurality of modulated frequencies.
For instance, in a case where a modulated frequency of 30 MHz and a modulated frequency of 300 KHz are used, a distance up to 5 m and a distance up to 500 m can be measured by each frequency respectively. In order to measure a distance longer than the above, further more frequencies are necessary. Conventionally, in order to measure a phase of a high-frequency wave, a heterodyne method is used, and phase measurement has been performed after decreasing the frequency. For instance, for the phase measurement of 30 MHz and 300 KHz, frequencies of 30 MHz−30 KHz and 300 KHz−30 KHz are generated, and phase measurement has been performed by the difference frequency of 30 KHz. When the frequency of 300 KHz−30 KHz, as generated at this moment, is used as a third modulated frequency, by obtaining a phase difference against the modulated frequency of 300 KHz, in a pseudo manner, the phase difference becomes equivalent to the phase modulated at 30 KHz, which made distance measurement up to 5 km possible.
In a conventional electro-optical distance meter, since a plurality of frequencies are prepared, distance measuring light is projected per each plurality of frequencies, and distance measurement is performed, there have been problems in that a circuit arrangement becomes complicated and measurement time becomes longer.